The Genetics of Hemophilia

A clotting disorder, hemophilia is caused by mutations to genes that regulate coagulation. Learn about the genetics of hemophilia in this article.

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What is Hemophilia?

Hemophilia is a genetically inherited disorder that disrupts the normal clotting process. Depending on how severe the hemophilia case is, the patient's blood may not be able to clot (changing from a liquid to solid state to prevent excess bleeding) after an injury. The Mayo Clinic states that hemophilia symptoms include unexplained bleeding or bruising, large or deep bruising, tightness in the joints, blood in the patient's urine or feces, nosebleeds without an apparent cause, and prolonged bleeding after an injury.

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Types of Hemophilia

The US National Library of Medicine states that the two main forms of hemophilia are hemophilia A and hemophilia B, which have similar symptoms but different mutated genes. Hemophilia A is also called classic hemophilia, and occurs in one out of 4,000 males worldwide; hemophilia is more common in males than females, due to the method of inheritance. Hemophilia B's moniker is Christmas disease, and affects one out of 20,000 males worldwide. The US National Library of Medicine adds that another form of hemophilia B can occur: hemophilia B Leyden, which results in excessive bleeding during childhood. The last type of hemophilia, acquired hemophilia, is not a genetic disorder; rather, it is caused by autoantibodies that disable coagulation factor VIII, which is involved in the clotting process.

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Hemophilia Genetics

The genetics of hemophilia involves several factors. Hemophilia A and hemophilia B affect different genes involved in clotting, according to the US National Library of Medicine: mutations in the F8 gene (hemophilia A) affect coagulation factor VIII, while mutations in the F9 gene (hemophilia B) affect coagulation factor IX. Both coagulation factors are responsible for the production of blood clots; when there is shortage of these proteins, the blood clots are not formed properly. The US National Library of Medicine notes that in severe hemophilia, the activity of either coagulation factor VIII or coagulation factor IX is completely shut down, resulting in no clot production. With mild or moderate hemophilia, however, the coagulation factor's activity is reduced, so clots are not made at the same efficiency as non-hemophiliacs.

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How is Hemophilia Inherited?

The US National Library of Medicine states that hemophilia is a X-linked recessive pattern, where the mutated gene is located on the X chromosome. This results in a higher frequency of occurrence in males, who only have on X chromosome; since females have two X chromosomes, they need two mutated genes to have the disorder, or else they become carriers. Carriers rarely have symptoms, though the US National Library of Medicine notes that 10 percent have mild problems with bleeding.

The National Heart, Lung and Blood Institute notes two different situations where hemophilia is inherited. In the first instance, the mother is a carrier and the father does not have hemophilia; their daughters have a 50 percent chance of being a carrier, and their sons have a 50 percent chance of being a hemophiliac. In the second instance, the father is a hemophiliac and the mother is not; their daughters have a 100 percent chance of being a carrier, and their sons have no chance of being a hemophiliac. This is because the father cannot pass a hemophilia gene to his sons, as the mutated genes are not linked to the Y chromosome.